The most widespread solution used for producing a re-built wafer consists first of all in cutting the wafers on which the chips have been fabricated to obtain individual chips, one wafer producing a first type of chips that are identical to one another, another wafer producing another type of chips, etc. A “chip” means an active electronic component such as a bare chip or a passive component (capacitors, resistors, transformers or inductors, etc.) or an MEMS (“Micro Electro Mechanical System”). Usually, these chips are then selected after having been tested and often called “Known Good Die”.
Moreover, the support designed to receive the tested chips is prepared: it is an adhesive rigid support.
The various types of tested chips, which have connection pads on a face called the active face or front face, are then picked out and positioned with the front face on the adhesive support by means for example of a “pick-and-place” machine for forming patterns of chips usually similar to one another. A chip pattern is a group of different chips designed to form an electronic element.
Then the chips are moulded in a polymer resin of the epoxy type which is then polymerized in order to render them fixed together.
The adhesive support is then removed.
A step of redistributing the pads of the chips on tracks intended for the future interconnection of the chips is then carried out by forming, on the front face of the chip, a redistribution layer or RDL usually itself comprising a stack of layers; in a known manner, the tracks of this RDL layer which are made of TiW/Cu or of TiPd/Au for example, are formed on a dielectric layer deposited in the place of the adhesive support, by dipping or by centrifuging.
It is possible to summarize these various steps with the following diagram:                1 Cutting the wafer to obtain individual chips        2 Preparing the adhesive rigid support        3 Transferring the cut chips to the adhesive rigid support        4 Moulding the chips in the resin and polymerizing        5 Removing the adhesive support        6 Redistributing the layers on the re-built wafer (RDL).        
The wafer thus re-built which contains no defective chips can then be cut into the patterns to obtain plastic micropackages; it can also be stacked onto other re-built wafers and electrically connected to these wafers according to various known methods, the stack then being cut to obtain three-dimensional or 3D electronic modules.
The encapsulation of the chips during step 4 comprises:                a step of depositing the resin (by pouring or by moulding called in compression), around and if necessary over the chips bonded to the adhesive support in order to fill the inter-chip spaces,        a step of polymerizing the resin in order to harden it and thus form a substrate that is rigid and handleable in which the chips are fixed, the adhesive substrate then being able to be removed.        
A well-known drawback is the movement of the chips during the depositing of the resin and/or during its polymerization with the result that the pads of the chips will no longer coincide with the tracks of the redistribution layer (RDL layer) during step 6. The micro-movements of the chips relative to the predicted position are due to:                A) the inaccuracy of positioning of the chips on the adhesive support, which is of the order of 5 μm with recent “pick-and-place” equipment operating at high speed,        B) the reversible but very high expansion of the adhesive support in the region of 100 ppm/° C.,        C) the irreversible removal of the resin during polymerization of the order of 1000 ppm/° C.,        D) the irreversible expansion of the resin after polymerization by approximately 16 to 17 ppm/° C.        
The result of this is micro-movements that are more or less isotropic and predictable, typically ranging between a few μm and a few tens of μm, which may exceed the tolerances of positioning (relative to the RDL layer) required after moulding, which are typically of the order of 10 μm.
One solution consists in studying and recording these micro-movements in advance and then anticipating them during the positioning of the chips when they are transferred to the adhesive support. One of the limitations of this technique arises from the fact that the micro-movements are not all predictable, notably those of the various types of chips within one and the same pattern.
Another solution consists in modifying the masks used for producing the RDL layer.